The present invention relates to an optical disc drive apparatus and, more particularly, is directed to an optical disc drive apparatus for use with a recording and reproducing apparatus using a phase-change type optical disc as a recording medium.
A recording and reproducing apparatus for recording and reproducing information signal with radiation of laser beams uses a disk-shaped recording medium (simply referred to hereinafter as "optical disc"). Such recording medium might be a read-only optical disc, a write-once optical disc in which an information signal can be written once and an optical disc using a magneto-optical recording medium in and from which an information signal can be recorded and erased a plurality of times.
The read-only optical disc includes a track on which uneven patterns, i.e., pits are formed concentrically or spirally formed on one surface thereof. Specifically, this optical disc is composed of a disc substrate made of a transparent synthetic material, such as polycarbonate, PMMA {poly (methyl methacrylate)} or the like, a reflection film made of a metal such as Al or Au so as to cover phase pits formed on one surface of the disc substrate and a protection film formed so as to cover the reflection film in order to protect the reflection film.
As the write-once optical disc, there is proposed a phase-change type optical disc which can form pits with high density. The phase-change type optical disc is composed of a transparent substrate in which pits are formed on the surface and a phase-change film formed on the transparent substrate. The phase-change film is at least one selected from Sb.sub.2 Se.sub.3, Sb.sub.2 Te.sub.3, Se, Te, BiTe, BiSe, In-Se, In-Sb-Te, In-SbSe, In-Se-Tb.sub.1, Ge-Te-Sb and Ge-Te.
A magneto-optical disc using a vertical magnetic recording medium or the like is known as a recordable optical disc.
This magneto-optical disc is composed of a disc substrate in which guide grooves for guiding laser beam are formed on one surface and which is made of a transparent synthetic resin such as polycarbonate, PMMA or the like, a recording layer made of a vertical magnetic recording material, such as Te, Fe, Co or the like, the recording layer being formed so as to cover the guide grooves, and a protection layer formed so as to cover the recording layer in order to protect the protecting layer.
A method of reproducing these optical discs will be described below. When the read-only optical disc and the write-once optical disc are reproduced, laser beam from a laser light source is radiated on the optical disc substrate under the condition that the laser beam is converged by an objective lens. Reflected light flux that was modulated by phase pits of the optical disc is detected by a photodetector, for example, and a reproduced signal of the information signal recorded on the optical disc is obtained by obtaining a detected signal having signal level corresponding to intensity of the reflected light flux.
When the latter recordable optical disc is reproduced, similarly to the read-only optical disc, laser beam from the laser light source is radiated on the disc substrate under the condition that the laser beam is converged by the objective lens. Then, a reproduced signal of the information signal recorded on the magneto-optical disc is reproduced by detecting a Kerr rotational angle in the reflected light flux modulated by the recording layer of the optical disc.
However, in the recording and reproducing apparatus, if data on the same track is periodically accessed at a cycle within a predetermined time, then it is frequently observed that the optical head is not returned to the home position so that the optical head stays on the same track. In this case, reproducing laser beam emitted from the optical head is radiated on the same track for a long period of time with the result that the track portion radiated by the reproducing laser beam is locally heated. There is then the risk that the optical disc itself is deformed by heat.
To solve the aforementioned problem, it is proposed to reduce a time ("predetermined time") in which the optical head is stopped on the same time. However, according to this proposed method, a frequency with which the optical head is moved to the home position is increased. There is then the risk that access efficiency is lowered.
Relationship between the frequency in which the optical head is moved to the home position and the efficiency in which the optical head accesses data is so-called trade-off relationship. Accordingly, it is not possible to satisfy both of the frequency and the access efficiency in response to all sorts of manners in which the recording and reproducing apparatus and the optical disc are in use.